1. Field of the Invention
This invention relates to the spectral analysis of rf signals, and more particularly to the analysis of signals having high frequencies considerably above 1 GHz.
2. Description of the Related Art
To obtain the spectral composition of a signal, it is necessary to sample the signal so that the highest frequency components are sampled at least twice each cycle (the Nyquist criterion). The higher the upper frequency one tries to detect, the shorter must be the time interval between samples. Unfortunately, electronic circuits are not presently capable of following very fast sampling rates. Other approaches have accordingly been tried to obtain the same result, such as the use of a sweeping electronic signal heterodyning with the unknown signal, and the use of acoustic-optic interactions. Acousto-optic modulators in particular are attractive because of their simplicity, fast real time operation, good resolution, large integration time and sensitivity. Such systems ar described, for example, in S. K. Yao and C. S. Tsai, "Acousto-optic Bragg Diffraction," Applied Optics, Vol. 16, (1977), p. 3032. Unfortunately, these systems are not capable of handling signal frequencies above several GHz.
U.S. Pat. No. 4,464,624 to Osterwalder describes a broad band optical spectrum analyzer in which a laser beam is reflected off a diffraction grating which reflects the beams due to different signal frequencies at different angles. The reflective rays are focused by a transforming lens so that rays with the same angular orientation are directed onto the same point. The intensities of the individual reflected spots are then taken as an indication of the frequency content of the original signal. This patent also discusses the general acoustic modulator system referred to above, in which the acousto-optical modulator is implemented as a Bragg cell. While claiming an operational range beyond 10 GHz, in actual practice the patented system exhibits a very small diffraction angle between different frequencies. This results in the need for a large apparatus, in the order of several meters long, to obtain a satisfactory resolution.
Another spectrum analyzer, referred to as the Lummer-Gehrcke interferometer, is described in a text by Born and Wolf, "Principles of Optics", 5th ed. (1975), Pergamon Press, pages 341-347. In this device a beam of light is admitted into a long, plane parallel plate of glass or crystalline quartz from a source on the long axis of the plate, through a prism fixed to one end of the plate. The beam meets the plate surfaces at an angle slightly smaller than the critical angle. A series of beams leaves each side of the plate near grazing emergence, and these are collected by a lens to form an interference pattern in the lens focal plane. The lens must be large enough to extend from one side of the plate to the other and collect the beams on each side.